This type of cell has a high theoretical energy density. These cells are, however, plagued by several serious problems, such as hydrogen evolution and dendrite formation at the zinc electrode a high rate of selfdischarge and storage of the free halogen formed during charge.
In order to counter these problems the development of these cells has almost exclusively gone in the direction of flow cells, where both anolyte and catholyte are circulated continuously and both compartments are separated by a separator or a cation exchange membrane. If the halide used is bromine, quaternary ammonium bases are dissolved in the electrolyte, which bind most of the bromine, which exists in solution in the form of polybromide ions. In the case of chlorine in most cases the chlorine formed during charging is stored as the hydrate at low temperatures and the cell has to be provided with a refrigeration system. From the above it is clear that these flow batteries are only suitable above a certain size, where it is economically feasible to provide the additional pumps, refrigeration systems and the like.
Several approaches to zinc halide cells have been described, but none of these cells has been successful.
Siegel et al., in U.S. Pat. No. 4,714,665, disclose a secondary battery comprised of three polymer films. The third film contains an electrolyte, and the first and second films contain an electrochemical species which may be in an anodic or a cathodic state, depending upon whether the battery is discharging or is being recharged. The third polymer sheet is substantially not electrically conductive under battery discharge voltages, but is electrically conductive under battery recharge voltages.
Zito, Jr., in U.S. Pat. No. 3,285,781, discloses a storage battery having specific separated compartments for each component of the cell, and a special plurality of compartments spaced relative to the positive electrode whereby bromine is distributed along that electrode for reaction, insuring intimate contact of the bromine an the positive electrode.
Noding et al., in U.S. Pat. No. 4,728,588, disclose a secondary battery composed of a polymer film and first and second collector plates. Only graphite is useful in this battery construction, as the use of carbon in other forms resulted in unsatisfactory batteries.
One of the major problems with zinc secondary cell batteries is the formation of dendrites. The dendritic growth which accompanies use of these types of batteries eventually penetrates the separator and causes short circuits. Heretofore, this problem has been dealt with by an overdesign in zinc capacity. However, this in turn reduces energy density.